In the fabrication of heat exchangers of the shell and tube type, especially those employed for use as vapor generators in nuclear power plants, it is the practice to assemble large numbers of closely spaced, small diameter tubes within a containment shell. In plants of large capacity currently being constructed the vapor generators employ several thousands of tubes of about 3/4 inch in diameter. Assembly of these tubes within the shell of effected by inserting the ends of the tubes into holes provided in a tube sheet that is formed as an integral part of the shell. The tubes are mounted to the tube sheet by expanding their ends into tight engagement within the walls of the holes. This expansive mounting may be accomplished by use of an explosive forming technique commonly referred to as "explansion".
Not infrequently, it becomes necessary to remove one or more of the tubes which has been explained into the tube sheet. This need may arise due to tube failure due to corrosion, for example, or also if a tube should become damaged during fabrication of the unit after having been mounted in the tube sheet.
In the past various types of induction heaters have been disclosed which may be inserted into a tube which is desired to be removed. Such heaters imparted induced expansive forces on the tube material which, due to the fact that the tube could not radially expand within the tube sheet, generated high stresses in the tube material. If the generated stresses exceeded the yield stress of the tube material, the material would be caused to expand axially such that, upon cooling, the tube would contract to a diameter sufficiently small to permit easy disassembly of the tube from the tube sheet. Such a device is shown and described in U.S. Pat. No. 3,492,453 to Kenneth Hurst. Induction heaters of this type suffered from the disadvantage that a relatively long period of time was required to adequately heat the tube. As a result, when a large number of tubes were to be removed, the down time of the unit was unacceptably long. A further disadvantages was that, while the shrinking of the tube was usually sufficient to permit removal from the tube sheet, it was not always sufficient to preclude damage to the tube sheet openings when the tubes were pulled out.
It is, therefore, a principle object of the present invention to teach a method and apparatus which will facilitate the removal of small diameter tubes which have been expandedly mounted within a tube sheet, in a relatively short period of time, with minimum damage to the tube sheet openings.